The present invention relates, for example, to a light beam scanning apparatus for scanning a plurality of laser beams in an image forming apparatus, such as a digital copying machine or a laser printer, which scans and exposes a single photosensitive drum with a plurality of laser beams at the same time, thereby forming a single electrostatic latent image on the photosensitive drum.
In recent years, various types of digital copying machines have been developed, which carry out image formation, for example, by scanning exposure with a laser light beam (hereinafter referred to merely as xe2x80x9clight beamxe2x80x9d) and an electronic photographing process.
Recently, in order to increase image formation speed, a digital copying machine adopting a multi-beam system has been developed. Specifically, in this system, a plurality of light beams are produced and a simultaneous scan is performed in units of a plurality of lines with these light beams.
The multi-beam type digital copying machine comprises a plurality of semiconductor lasers for producing light beams, a polyhedral rotary mirror, such as a polygon mirror, for reflecting the light beams output from the plural lasers toward a photosensitive drum to scan the photosensitive drum with the light beams, and an optical system unit functioning as a light beam scanning apparatus composed mainly of a collimator lens and an f-xcex8 lens.
In the conventional multi-beam type digital copying machine, in order to produce a high-quality image, the optical system unit controls an exposure position in a scan direction of a light beam (main-scan-direction beam position control) and a beam passage position (sub-scan-direction beam position control).
Examples of the control technique are disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 1-43294, Jpn. Pat. Appln. KOKOKU Publication No. 3-57452, Jpn. Pat. Appln. KOKOKU Publication No. 3-57453, Jpn. UM Appln. KOKOKU Publication No. 5-32824, Jpn. Pat. Appln. KOKAI Publication No. 7-72399, Jpn. Pat. Appln. KOKAI Publication No. 7-228000, Jpn. Pat. Appln. KOKAI Publication No. 9-210849, Jpn. Pat. Appln. KOKAI Publication No. 9-258125, Jpn. Pat. Appln. KOKAI Publication No. 9-314901, and Jpn. Pat. Appln. KOKAI Publication No. 10-76704. The techniques disclosed in these documents have problems, as will be described below.
In the main-scan-direction beam position control, it is important that a sensor, or sensor means, be disposed with a predetermined relation (angle) to the beam scan direction. If the sensor is displaced from a predetermined position, it cannot exactly sense the main-scan-direction beam position (relative position). As a result, for example, a vertical straight line cannot be formed.
However, Jpn. Pat. Appln. KOKAI Publication No. 9-314901 alone discloses the technique wherein the sensor itself has the function of sensing the relation between the sensor and the light beam scan direction. Even in this case, there are problems in that the range of sensing of inclination is very narrow and the sensing and adjustment are not easy.
As regards the sub-scan-direction beam position control, Jpn. Pat. Appln. KOKAI Publication No. 7-72399, Jpn. Pat. Appln. KOKAI Publication No. 7-228000 and Jpn. Pat. Appln. KOKAI Publication No. 9-210849 disclose examples wherein the beam passage position in the sub-scan direction is sensed on the basis of the length of time over which the light beam passes by the optical sensor.
However, where a variance occurs in f-xcex8 characteristics of an f-xcex8 lens mounted on the optical system unit or a variance occurs in the number of rotations of the polygon mirror, the scan speed of the light beam over the sensor will vary. Consequently, a sensing error may occur in these sensing methods based on the light beam passage time.
Jpn. Pat. Appln. KOKAI Publication No. 9-258125, Jpn. Pat. Appln. KOKAI Publication No. 9-314901 and Jpn. Pat. Appln. KOKAI Publication No. 10-76704 disclose examples wherein the light beam passage position is made to fall between specific sensor patterns formed on the sensor, whereby the light beam passage position is controlled to fall on a predetermined position. In the structures described in these documents, however, it is necessary to control the light beams individually so as to fall on predetermined positions. Compared to the case where a certain light beam is used as a reference beam and the passage positions of the other light beams are controlled on the basis of the reference beam, the number of actuators for controlling the beam passage positions increases and the cost rises.
Moreover, the sensor patterns for making the light beams fall on predetermined positions have high sensing precision, but the range (=sensing range) of the variation in sensor output relative to the variation in beam passage position is narrow. Consequently, control becomes complex, and much time is required for control.
An object of the present invention is to provide a light beam scanning apparatus capable of exactly detecting a passage position of a light beam in a wide range in a sub-scan direction using a less number of sensor patterns, and performing a high-precision beam passage position control.
Another object of the invention is to provide a multi-beam scanning apparatus capable of exactly controlling relative passage positions of a plurality of light beams in a wide range in a sub-scan direction using a less number of sensor patterns.
Still another object of the invention is to provide a multi-beam scanning apparatus capable of controlling passage positions of light beams to come to predetermined positions using a less number of actuators than the number of simultaneously scanned light beams.
Still another object of the invention is to provide a light beam scanning apparatus capable of exactly performing a desired beam passage position control, without the effect of an eclipse of a light beam.
Still another object of the invention is to provide a light beam scanning apparatus with a simplified structure, which is attained by sharing of sensor patterns.
Still another object of the invention is to provide a light beam scanning apparatus capable of determining whether a beam passage position control is feasible or not when a passage position of a light beam has varied due to a variation with the passing of time, and issuing an alarm where the beam passage position control is not feasible.
Still another object of the invention is to provide a light beam scanning apparatus capable of matching with an increase in the light beam scanning speed.
A pair of sensor patterns for detecting the beam passage position according to the present invention are juxtaposed in the main scan direction and are constructed such that an output from one of the paired sensor patterns successively increases while an output from the other sensor pattern successively decreases in relation to a variation of the passage position of the light beam in the sub-scan direction. The paired sensor patterns have a sufficient length in the sub-scan direction and can exactly detect the passage position of the light beam in a wide range in the sub-scan direction. Therefore, with the simple structure and operation, a high-precision beam passage position control can be performed.
The paired sensor patterns according to this invention are applicable to a light beam scanning apparatus using a multi-beam optical system which simultaneously scan a plurality of light beams. Therefore, relative passage positions of plural light beams can exactly be controlled in a wide range in the sub-scan direction.
A sub-scan-directional position of a first beam of the plural light beams may be fixed. Therefore, passage positions of light beams can be controlled to come to predetermined positions using a less number of actuators than the number of simultaneously scanned light beams.
Light amounts of a plurality of light beams are detected at a position equivalent to a position of the surface to be scanned. Using a correction coefficient corresponding to a ratio of the detected light amounts, outputs from the sensor patterns for beam passage position detection are corrected. Therefore, a desired beam passage position control can exactly be performed without the effect of an eclipse of a light beam.
The outputs from the paired sensor patterns according to the present invention are added to detect the light amount of the light beam. Specifically, the sensor patterns for the beam passage position detection are also used for the light amount detection. Therefore, the light beam sensor and the process circuit for the output thereof can be simplified, and the degree of freedom of arrangement of other sensor patterns is increased. A nearly optimal arrangement of sensor patterns can be made.
Sensor patterns are provided to monitor the passage position of the fixed light beam. Where the passage position of the fixed light beam has varied due to a variation with the passing of time, it is determined whether the beam passage position control is feasible or not. If it is determined that the beam passage position control is not feasible, an alarm is issued to indicate this undesirable state.
In the present invention, unlike the prior art, the sensor pattern output integration time period in the beam passage position detection mode may be different from that in the offset detection mode. On the basis of the ratio of the integration time periods, the detected offset value is corrected. As a result, the degree of freedom of arrangement of sensor patterns is increased.
With the above features, in the present invention, in particular, in the field of image forming apparatuses such as digital copying machines, the quality of output images can be enhanced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.